Manchester United may have to give up their interest in Gareth Bale and Cristiano Ronaldo, Real Madrid their pursuit of United keeper David de Gea and Real their immediate interest in Eden Hazard after the Bernabeu club and city rivals Atletico Madrid were handed a transfer ban by Fifa for the next two windows.
Both clubs were found guilty of breaching rules over the transfer of under-18 players, with Atletico also fined SFr900,000 (£622,000) and Real told to pay SFr 360,000. Both clubs deny any wrongdoing and say they will appeal.
Although the ban does not cover the current January transfer window, it could have a far-reaching impact on the global transfer market over 2016 and 2017 as the two clubs are prevented from signing any players in next summer's window or the January 2017 window.
The two clubs can sell players, but there would seem no point because they would not be able to sign any replacements. They can also buy players, but could not play them until the ban ends. The only way round the ban would be for Real and Atletico to fast-track all their planned transfer business and buy players in the remaining two weeks of this transfer window.
The ramifications of the ban could be huge. De Gea, will have to stay put. He nearly joined Real last summer, and his new contract is reported to have a buy-out clause which would have allowed him to join his home-town club.
Hazard would also be affected. He has been interesting Real for some time, just as United have been considering Bale and Ronaldo, who also reportedly interests Paris Saint-Germain.
However, it is inconceivable that they would let their big names leave if they could not replace them.
Equally, there would be little chance of Chelsea or United persuading Atletico Madrid to sell them French winger Antoine Griezmann if Atletico could not sign anyone to replace him.
In a statement, Fifa gave the reasons for the ban, saying: "The two clubs were found to have violated several provisions concerning the international transfer and first registration of minor players, as well as other relevant provisions with regard to the registration and participation of certain players in competitions."